The specific aim of this project is to study factors that influence lung function of infants at birth and during the first 12-18 months of life. Specifically, the proposal will: 1) study the effect of material cigarette smoking habits during pregnancy on lung function at birth and the development of function of the first 12-18 months; 2) study the effects of postnatal factors such as acute respiratory illness, continued exposure to maternal and other sources of cigarette smoke in postnatal lung function development; 3) study the extent to which the reported increased frequency of respiratory illness in infants of smoking mothers is the result of altered lung mechanics which result from in utero exposure to products of tobacco smoke and/or postnatal passive exposure to maternal or other sources of tobacco smoke; 4) study the extent to which the effect of pre-and postnatal exposure to maternal cigarette smoking on lung function is mediated by an increased frequency/severity of acute respiratory illness. Pregnant women will be identified during their first trimester and followed monthly with assessments of smoking habits, urine cotinine, general medical condition and status of the pregnancy. Within two weeks of birth, infants will be evaluated with partial forced expiratory flow-volume curves and measurement of passive respiratory system compliance and resistance. Measurements will be repeated at 4, 10 and 15 months. Babies will also be observed regularly in terms of respiratory illness experience (active and passive surveillance), passive exposure to cigarette smoke (history from mother, urine cotinine from infant) and general respiratory health (symptoms, specific illnesses). The analysis of the data will focus on the factors which influence the levels of lung function at birth and the rate and pattern of growth during the first 18 months of life. Techniques of time-series analysis (autoregressive models) will be used to explore the longitudinal character of the data in terms of the interplay between prenatal and postnatal exposures. The proposal seeks, through the application of a combiation of physiologic and epidemiologic techniques, to identify the means by which passive exposure to cigarette smoke alters respiratory health and the points in time when intervention would be most likely to minimize the possibility of irreparable harm.